This invention relates generally to a process of using microorganisms to convert hydrocarbons (liquids, solids, and gases) to methane and other gases in a subterranean formation.
When oil is present in subterranean rock formations such as sandstone, carbonate, chert or shale, it can generally be exploited by drilling into the oil-bearing formation and allowing existing pressure gradients to force the oil up the borehole. This process is known as primary recovery. If and when the pressure gradients are insufficient to produce oil at the desired rate, it is customary to carry out an improved recovery method to recover additional oil. This process is known as secondary recovery.
There are several secondary recovery techniques, including gas injection and water injection. Choice of a specific secondary recovery technique depends on the specifics of the hydrocarbon accumulation. Water injection or waterflooding is the most common secondary recovery technique. In waterflooding, pressurized water is injected into the oil-bearing formation and produced from neighboring hydrocarbon production wells. First hydrocarbon, and subsequently hydrocarbon and water are recovered from the production well.
However, even after secondary recovery, a significant portion of hydrocarbon remains in the formation, in some cases up to 75% of the original hydrocarbon in place. The fraction of unrecoverable hydrocarbon is typically highest for heavy oils, tar, and complex formations. In large oil fields, more than a billion barrels of oil can be left after conventional waterflooding. Much of this remaining oil is in micro-traps due to capillary forces or adsorbed onto mineral surfaces (irreducible oil saturation) as well as bypassed oil within the rock formation. One enhanced oil recovery technique uses microorganisms such as bacteria to dislodge the micro-trapped or adsorbed oil from the rock. The goal of this technique, which is known as microbially enhanced oil recovery (MEOR), is to increase oil recovery of the original subsurface hydrocarbons. MEOR processes typically use microorganisms to: (1) alter the permeability of the subterranean formation, (2) produce biosurfactants which decrease surface and interfacial tensions, (3) mediate changes in wettability, (4) produce polymers which facilitate mobility of petroleum, (5) produce low molecular weight acids which cause rock dissolution, and (6) generate gases (predominantly CO2) that increase formation pressure and reduce oil viscosity.
Numerous microorganisms have been proposed for achieving various microbial objectives in subterranean formations. Most MEOR techniques involve injection and establishment of an exogenous microbial population into the oil-bearing formation. The population is supplied with nutrients and mineral salts as additives to the waterflood used for secondary oil recovery. The development of exogenous microorganisms has been limited by the conditions that prevail in the formation. Physical constraints, such as the small and variable formation pore sizes together with the high temperature, salinity and pressure of fluids in the formation and the low concentration of oxygen in the formation waters severely limit the types and number of microorganisms that can be injected and thrive in the formation. Biological constraints, such as competition from indigenous microbes and the stress of changing environments (from surface to subsurface) also act to limit the viability of exogenous microorganisms. To overcome these problems, indigenous microorganisms, commonly anaerobic, have been proposed in MEOR projects.
In addition to MEOR activities, microorganisms have also been proposed for making methane from fossil fuel deposits. Recent studies have demonstrated that hydrocarbons can be converted by microbial processes to methane. Patents proposing this concept include U.S. Pat. No. 3,826,308 (Compere-Whitney), which used microorganisms to produce methane from fossil fuel deposits containing organic ring compounds; U.S. Pat. No. 5,424,195 (Volkwein) which converted coal to methane in situ using a consortium of exogenous microorganisms (developed in a coal-bearing cavity); and U.S. Pat. Nos. 4,845,034 and 4,826,769 by Menger et al. which pretreated subterranean cavities having finely-divided carbonaceous material with an aqueous alkali solution and then biochemically reacted the carbonaceous material with either exogenous or indigenous methane-producing microorganisms to produce methane. The prior art does not suggest how to identify specific microorganisms of the indigenous microbial community for the purpose of determining processes for stimulating and sustaining microbial activity to degrade hydrocarbons and generate methane. The prior art also does not suggest developing a practical ecological model to determine processes for stimulating and sustaining in situ microbial activity.
With appropriate environmental conditions and sufficient time, indigenous bacteria can convert hydrocarbons to methane. However, such a natural process may require residence times of hundreds to millions of years. The conversion process is apparently slow under most geological conditions. The problem to be solved is to find a process of accelerating and sustaining biochemical conversion of hydrocarbons to methane at a rate that is commercially practical or establishing and/or maintaining an in situ environment that supports commercial rates of hydrocarbon conversion and methanogenesis.
This invention relates to a process of stimulating the activity of microbial consortia in a hydrocarbon-bearing, subterranean formation to convert hydrocarbons to methane and other hydrocarbon gases, which can be produced. The hydrocarbons can be carbonaceous deposits in solid, liquid, or gaseous form such as coal, oil shale, tar sands, oil formations, and rich gas or the hydrocarbons can be unwanted subsurface hydrocarbons of a hydrocarbon reclamation project. An analysis is made of the environmental conditions in the formation, preferably by obtaining samples of formation fluid and/or rock and then analyzing the samples. The presence of microbial consortia in the formation is determined, preferably by analyzing one formation samples for the presence of microorganisms in the samples. A characterization, preferably a genetic characterization, is made of at least one microorganism of the consortia, at least one of which is a methanogenic microorganism, and comparing said characterization with at least one known characterization, preferably a genetic characterization, derived from a known microorganism having one or more known ecological characteristics. This information, together with the information obtained from the analysis of the fluid and rock, is used to determine an ecological environment that promotes in situ microbial degradation of formation hydrocarbons and promotes microbial generation of methane by at least one methanogenic microorganism of the consortia. This ecological information is then used as the basis for modifying the formation environment to stimulate and sustain microbial conversion of formation hydrocarbons to methane. The formation environment can be modified by carrying out at least one of the following stimulation techniques: (1) adding, subtracting, and/or maintaining components needed for microbial growth, and/or (2) controlling and/or maintaining formation environmental factors such as chemistry, temperature, salinity, and pressure.